Field of the Invention
The present invention relates to a system for obtaining analyte measurements. Specifically, the present invention relates to an external reader that can interrogate an implanted analyte sensor.
Description of the Background
In a system in which an external sensor reader provides power to an implanted sensor for operation (e.g., analyte measurement) and data transfer, the primary coil of the external sensor reader must be appropriately aligned with the secondary coil of the implanted sensor. However, there is a finite and relatively short range (typically less than one inch) within which the implanted sensor receives an electrodynamic field from the external sensor reader of sufficient strength to power the sensor for analyte measurement and data transfer. In addition, finding the correct alignment is made more difficult because an implanted sensor is not visible to the user.
An attached system that physically maintains the external sensor reader in alignment with the implanted sensor using, for example, a fixed wristwatch, armband, or adhesive patch does not work well for users who do not wish to wear a wristwatch, armband, adhesive patch, or other fixed system and/or only require intermittent readings from the implanted sensor during any period in time. Furthermore, even a system that enabled on-demand measurement would be unsatisfactory if it required a user to probe around either by trial and error or by watching a field strength meter to find the relative position in space from which to initiate a reading.
RFID systems and readers are used for animal identification, anti-theft applications, inventory control, highway toll road tracking, credit card, and ID cards, but are not applicable in the context of an implantable sensor and external reader system. RFID systems are transponders, and the energy supplied must only reflect a preset numerical sequence as an ID. This requires much less power than an activated remote/implanted sensor, and an RFID system is therefore capable of much more range because of the extremely low operational power requirement from the RFID tag and can allow operation at ranges of up to 5 feet or more. In contrast, an implanted analyte sensor must be provided with much more power to operate its circuitry for making measurements and conveying the data to the reader. In fact, transfer of power by induction between two coils is very inefficient at distance, and such systems are often limited to approximately one inch or less, instead of multiple feet possible in RFID systems.
A hobby or utility grade metal detector or stud finder is also inapplicable in the context of an implanted sensor and external reader system. Metal detection or stud finding is an example of motion type operation, but the relationship between the primary coil and the metal to be detected is completely passive. Thus, in stark contrast with an implanted sensor and external reader system, where the implanted sensor requires power for activation, measurement, and data transfer, no power is required to activate the metal being detected by a metal detector or stud finder, and only the relative motion perturbation of the electromagnetic field is required.
Accordingly, there is a need for an improved implanted sensor and external reader system.